High performance cross-linked poly(2-acrylamido-2-methylpropanesulfonic acid)-based proton exchange membranes for fuel cells

Poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) has been used for the preparation of proton exchange membranes (PEMs) due to the superior ability of sulfonic acid groups originated from AMPS in supporting proton conduction. Since PAMPS is water-swollen or even soluble in water, it is necessary to copolymerize AMPS with proper monomer oils to control the swelling and mechanical properties of membranes. However, as a water-soluble monomer, AMPS is generally immiscible with hydrophobic monomer oils. To overcome this obstacle, AMPS was converted to an ionic liquid (IL) form by neutralization with a Bransted base, 1-methylimidazole. The resultant 1-methylimidazolium 2-acrylamido-2-methylpropanesulfonate ([MIm][AMPS]) is miscible with monomer oils and thus could be cross-linked with styrene, acrylonitrile, and divinylbenzene. The produced copolymer membranes in IL form were converted to acid form by treated with acid. Membranes in both IL and acid forms have good thermal stability and mechanical properties which could be further tuned by the content of cross-linking agent. The water uptake, swelling degree, proton conductivity, and methanol permeability of the membranes increased with the PAMPS content while decreased with cross-link agent content. The acid form PAMPS-based membranes showed high proton conductivity of 3.82×10-2S/cm and low methanol permeability of 3.80×10-7cm2/s at 25 °C. The properties of the samples suggested that this type of cross-linked PAMPS-based membranes were particularly promising to be used as PEMs in direct methanol fuel cells.